Restructure the data store.

 - Facades: Transaction and Data Store
 - Internal: Transaction is a stack of operations, DataStore is a stack of transactions.
 - When client code commits or rolls back transaction, facade forwards to real transaction, which tells the internal data store to apply the operations
 - Move the logic for each action into the operation so that we fully utilise polymorphism

All tests pass. But I'm quite certain the tests are insufficient...

You'll also need to add virtual dtors to the following classes:
 - You::DataStore::Internal::IOperation
 - You::DataStore::Internal::PostOperation
 - You::DataStore::Internal::PutOperation
 - You::DataStore::Internal::EraseOperation
Still no clue of what the line means. I mean, `operator` is a variable right? (It is used in transaction.cpp) But, of what type? And why does it have some weird operation? (like `operator->()->someMethod()`?! What does that even mean?)